Is it immature to ignore an ex?

My ex texted me yesterday telling me about a party at our friends house (which I already knew about & I'm sure he knew that I did too) then later after I didn't respond texted me "how are you"

We broke up last Sunday. I didn't go to the party & haven't responded but now I feel rude & immature.. I don't want to get into anything with him (he broke it off) but I do love him (always will, known him forever) so I was thinking of responding as "I'm sorry, I just can't"

Not sure what to do
Is it immature to ignore an ex?
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